The present invention is directed, in general, to a resistor and, more specifically, to a resistor for use in an analog circuit and a method of manufacturing such a resistor.
The desire for clearer and more efficient means of signal transmission and reception has prompted a rapid movement toward the use of digital circuits within various transceiver devices, such as digital mobile phones. One industry that uses such devices is the telecommunications industry, which employs integrated circuits in conjunction with analog-to-digital and digital-to-analog converters with its devices. These converters perform a vital function, by converting signals into their proper analog or digital format. A critical element of the analog device is a series of resistors that enables the analog device to precisely perform the conversion. Ironically, however, these very resistors are often the source of reduced reception and transmission quality. As it is well known, for an analog circuit to function properly, a very precise and consistent resistance value is required. If the value of the resistance changes in some way, the device""s transmission or reception quality is diminished, which can result in signal break up or signal distortion.
Due to the large demand for high quality transmission and reception devices, the semiconductor industry has developed methods of manufacturing analog circuits and incorporating them into or associating them with integrated digital circuits. As previously mentioned, analog circuits require a resistor containing a precise resistance value. Moreover, it is highly desirable that resistors can be manufactured to have predetermined or tailor-made resistance values for differing applications. It is very important, however, that the resistance remains substantially constant through various temperatures and operating voltages. Thus, resistors that have a predetermined resistance and that are made of materials having low temperature and voltage coefficients are desired to accommodate the precision needed in manufacturing analog circuits.
One such resistor that is commonly used in analog circuits is the diffused resistor. Diffused resistors are resistors that normally comprise a poly silicon (Poly-Si) or silicon (Si) material and that have a dopant diffused therein to give it the desired resistance. Such resistors are doped to achieve the desired resistance value, which is controlled by the length and width of the resistor used, depth of diffusion and the resistivity of the dopant used. The use of diffused resistors is popular because of their compatibility with the remainder of the semiconductor manufacturing process. That is, they can be formed at the same time as the other circuit elements, and hence, do not add to the fabrication cost. Furthermore, they are desirable because they may be tailored to the space available and the resistivity needed.
Diffused resistors are not without their problems, however. For example, the resistance value of diffused resistors can change as the result of subsequent processing temperatures, operating temperatures or applied operating voltages. In addition, diffused resistors may also suffer from parasitic junction capacitance associated with the resistor and the underlying region, and exhibit high temperature and voltage coefficients. Thus, while a diffused resistor""s resistance can be tailored for a specific application, its resistance may change depending on the temperature or operating voltage, which can lead to diminished transmission or reception quality.
To circumvent the drawbacks associated with diffused resistors, thin-film deposited resistors, such as thin metal film transistors, have also been used. The resistance of a thin-film resistor is dependent on length and area of the resistor device, and the naturally occurring resistivity of the metal being used. Thin-film resistors are often used because their resistance can be determined quickly and precisely. Additionally, they have low temperature and voltage coefficients, allowing them to maintain a precise resistance value during fluctuations in operation temperature and voltage. Furthermore, thin-film resistors exhibit smaller parasitic capacitance values than do diffused resistors. Unfortunately, however, thin-film resistors also have their drawbacks. Thin-film resistors have the resistivity value of the material of which they are made, and as such, their resistance cannot be tailored to specific applications.
Accordingly, what is needed in the art is a resistor and method of manufacturing therefore that can be precisely tailored for use in a telecommunications device and that does not experience the problems associated with the diffused and thin-film resistors. The present invention addresses these needs.
To address the above-discussed deficiencies of the prior art, the present invention provides a unique resistor formed on a semiconductor substrate. In one embodiment, the resistor comprises a first resistor layer that includes a metal silicide, such as tungsten silicide, and nitrogen and that is formed on the substrate. The first resistor layer has a first thickness and a concentration of nitrogen incorporated therein. Preferably, the concentration of nitrogen ranges from about 0.1% to about 30%. However, the nitrogen concentration may be varied to obtain a desired resistive value of the resistor. Thus, depending on the concentration of nitrogen, a wide range of resistive values may be achieved. However, in preferred embodiments, the resistive value may range from about 10 ohms/sq. to about 1000 ohms/sq. The resistor further comprises a second resistor layer with a second thickness that includes the metal silicide and that is formed on the first resistor layer.
Thus, in a broad scope, the present invention provides a metal silicide-based resistor having nitrogen incorporated therein which allows the resistance of the resistor to be tailored to specific electrical applications. Yet, at the same time the resistor is far less susceptible to temperature and voltage variation than conventional diffused resistors have been and, thereby, provides a more precise resistor.
In one embodiment, the resistance of the resistor may be varied as a function of a ratio of the first thickness to the second thickness, and relative to the nitrogen concentration. In such embodiments, the ratio may range from about 1:1 to about 1:5. In a more preferred embodiment, however, the ratio is about 1:3.
In another embodiment, the present invention provides a digital-to-analog converter. In this particular embodiment, the digital-to-analog converter includes a digital input that receives a sequence of bits representing a number, an analog conversion circuit that receives the sequence of bits from the input and associates an electrical characteristic with the sequence of bits based on values thereof. The previously described resistor, or its various above-described embodiments, forms a part of the analog conversion circuit. The resistor may be incorporated into the analog conversion circuit or separate from, but electrically connected to it. Preferably, the resistor forms part of a ladder network in the analog conversion circuit. This particular digital-to-analog circuit also includes a summing circuit, which in certain embodiments may include an operational amplifier, that adds the electrical characteristics to generate an analog value that is equivalent to the number.
In another aspect, the present invention provides an integrated circuit located on a semiconductor substrate. This particular embodiment, of course, comprises transistors formed on the substrate, interconnect structures formed on multiple levels within the integrated circuit that electrically connect to the transistors in a pattern to form the integrated circuit, the digital-to-analog converter as previously described, and a summing circuit that adds the electrical characteristics to generate an analog value that is equivalent to the number.
In yet another aspect, the present invention provides a method of forming a resistor on a semiconductor substrate. In a preferred embodiment, the method includes forming a first resistor layer including a metal silicide and nitrogen on the substrate. The first resistor layer has a first thickness and a concentration of nitrogen incorporated therein, which is dependent on a desired resistive value of the resistor. The method further includes forming a second resistor layer, which has a second thickness, including the metal silicide, as on the first resistor layer.
In one embodiment of the above-described method, a physical vapor deposition process is used to fabricate the resistor. In such embodiments, the method includes forming a first layer includes forming the first resistor layer in a presence of nitrogen gas, having a flow rate ranging from about 5 sccm to about 100 sccm. As with previous embodiments, forming the first and second resistor layers includes forming the first and second resistor layers to a thickness wherein a ratio of the first thickness to the second thickness ranges from about 1:1 to about 1:5. More preferably, the ratio is about 1:3. The metal silicide used to form the first and second resistor layers may be tungsten silicide. However, other metals known to those who are skilled in the art may also be used.
The concentration of nitrogen may vary depending on the resistor""s desired resistive value. However, in a preferred embodiment, the first resistor layer may be formed such that it has a nitrogen concentration that ranges from about 0.1% to about 30%, or, it may be formed such that the resistor has a resistive value that ranges from about 10 ohms/sq. to about 1000 ohms/sq.
In another embodiment, the method may further comprise forming an integrated circuit on the substrate, including forming transistors on the substrate and forming interconnect structures on multiple levels within the integrated circuit to electrically connect the transistors in a pattern to form the integrated circuit. In yet another aspect of this particular embodiment, the method may further include forming a digital-to-analog converter that is electrically connected to the integrated circuit, including forming a digital input that receives a sequence of bits representing a number, forming an analog conversion circuit that receives the sequence of bits from the input and associates and electrical characteristic with the sequence of bits based on values thereof, the analog conversion circuit including the resistor, and forming a summing circuit that adds the electrical characteristics to generate an analog equivalent of the number.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.